“You know, there’s a reason we haven’t had a woman president before.”

President Barack Obama on Tuesday called on men to “look inside” themselves and think about bias if they have any doubt about voting for Democratic nominee Hillary Clinton.

“To the guys out there, I want to be honest,” Obama told a crowd in Columbus, Ohio, at a Clinton campaign event. “You know, there’s a reason we haven’t had a woman president before.”

He asked male voters to think about their potential biases against Clinton, who would be the first female president in U.S. history.

“I want every man out there who’s voting to kinda look inside yourself and ask yourself, if you’re having problems with this stuff, how much of it is that we’re just not used to it?” he said. “When a guy is ambitious and out in the public arena and working hard, well that’s okay. But when a woman suddenly does it, suddenly you’re all like, well, why’s she doing that?”

Obama went on to praise Clinton. “She is so much better qualified than the other guy,” he said, referring to GOP nominee Donald Trump. “She has conducted herself so much better in public life than the other guy.”

Obama has spoken out before about the sexism Clinton has faced during the 2016 presidential election.

In an interview with Samantha Bee on Monday, Obama said that positive attributes in men, such as ambition, are often seen as negative qualities in women. And even if Clinton becomes president, Obama said he expects she will continue to face sexism during her time in office.

For Americans who are on the fence about who to vote for, Obama told Bee that the choice “could not be clearer.”

“If we want to build on progress on issues like climate change and gender equality and making sure that everybody has health care and making sure that young people have a good education and can afford college, they’ve gotta make sure that their voices are heard.”